Proton exchange for high-index waveguides in LiNbO3

Abstract

We describe the fabrication and characterization of optical waveguides formed in LiNbO3 by proton exchange in benzoic acid melts at 200–250 °C. Proton exchange, in LiNbO3 the replacement of lithium ions with protons, takes place when the substrate is immersed in the molten acid. We observe a surface increase in the refractive index of 0.12, for the extraordinary polarization only, with a step function index profile. This is the highest index increase obtainable to date for LiNbO3. Measured diffusion rates for x-cut crystals are 1.37 μm2/h at T = 249 °C and 0.37 μm2/h at T = 217 °C, so that very deep guides can be formed in short times. Diffusion is somewhat slower in the z direction. The process as described is not useful for y-cut crystals, since it damgaes this surface. Losses, measured on x-cut samples, were <0.5 dB/cm. All measurements were made at 0.633 μm.

Nanoscale characterization of β-phase HxLi1−xNbO3 layers by piezoresponse force microscopy

“…4,8 Proton exchange (PE) is a process relying on the controlled introduction of protons in LN via a thermally activated ion-exchange (H þ $Li þ ). 9 It was originally devised for integrated optics, 10 but it also opens up possibilities for domain engineering and ferroelectric lithography on LN substrates. 5,8,11,12 Conventional characterizations of PE in LN, made by optical techniques 13,14 are inherently limited in their spatial resolutions by beam cross-sections (in the micrometric range), while alternative methods, which can enable nanoscale imaging, are often cumbersome and destructive.…”

Fabrication and characterizations of proton-exchanged LiNbO3 waveguides fabricated by inductively coupled plasma technique

“…12 Due to strong lattice deformations, PE waveguides are particularly difficult to obtain on Y-cut LN via the benzoic acid ͑BA͒ or similar molten acid processes. 13,14 Although it was possible to obtain reasonable PE layers at relatively low temperatures ͑up to ϳ160°C͒ for short exchange times ͑up to ϳ60 min͒, damage started to occur in the LN surface when the temperature was increased or the exchange time was prolonged in order to obtain a deeper PE layer.…”

UV laser-induced poling inhibition in proton exchanged LiNbO $$_{3}$$ 3 crystals

“…The integrity of the PE waveguide in the vicinity of the laser irradiated tracks was investigated by optical transmission experiments. The PE:CLN sample under investigation has been uniformly proton exchanged on its +z-face to form a planar waveguide that can support TM mode [12,19]. The UV laser tracks were exposed along the y-direction of the crystal on the +z-face.…”

“…Proton exchange is a well-established method for the fabrication of low-loss waveguides, which are resistive to photorefractive damage [11]. PE waveguides are formed by the replacement of lithium ions with protons from a benzoic acid melt (up to a depth of ∼1 µm), which increases the extraordinary refractive index of the crystal [12,13]. The main question that this work attempts to address is whether the UV laser irradiation will produce sufficient coercive field contrast for selective poling, as in the case of poling inhibition of undoped crystals [6,7].…”